Field of the Invention
The invention relates to a sensor element which includes a substrate having a strain-sensitive element preferably applied by means of thin-film technology, which element is used for measuring the deformation of the substrate when pressure is applied or a force is introduced.
The Prior Art
In order to measure static pressures for example, conventional sensor concepts which are mostly based on membranes use piezoresistive methods or so-called strain gauges (SG) in order to enable the detection of local mechanical strains in a highly precise way. Conventional SG materials (such as CrNi) cannot be used for high-temperature applications (up to 700° C.) because their electromechanical properties change irreversibly after high-temperature loading or show a strong tendency towards oxidation. Metallic materials with a high melting point such as Pt (see “A robust pressure sensor for harsh environmental applications”, S. Fricke, A. Friedberger, H. Seidel, U. Schmid, Sensors and Actuators A: Physical, Volume 184, September 2012, Pages 16-21) can principally be used up to the indicated temperature range, but they have a very high temperature coefficient of electrical resistance (TCR), which requires the highest precise temperature measurement on the sensor element which can hardly be realised technically over the entire temperature range.
The object is thus the development of thin-film systems with high temperature stability, a TCR close to zero and the highest possible K factor, which is a measure for the change in the electrical resistance under mechanical strain.
Ni:a-C:H thin films fulfil the latter two requirements, but they are only stable up to approximately 300° C. Such layers are described for example in “Pressure sensitivity of piezoresistive nickel-carbon Ni:a-C:H thin films”, Steffen Uhlig, Hanna Schmid-Engel, Tobias Speicher, Günter Schultes, Sensors and Actuators A: Physical, Volume 193, 15 Apr. 2013, Pages 129-135.
In this context, EP 1 991 849 B1 discloses a high-temperature pressure sensor element for measuring pressures even above 400° C. The high-temperature pressure sensor element or a pressure sensor consists of a substrate in which an inner region is configured. Furthermore, a deformable membrane is provided, which in operation separates the inner region from the outer region and deforms during a change of the external pressure in relation to the pressure in the inner region. An arrangement or a structure of strain gauges is arranged on the deformable membrane, which strain gauges form a strain gauge element for measuring the deformation of the membrane. The membrane, the substrate and the strain gauge element are formed from one and the same high-temperature-stable material, preferably an alloy such as a nickel base alloy. An electrical insulating layer is arranged between the strain gauge element and the membrane, which insulating layer consists of BN, MgO or Al2O3. The strain gauge element can further comprise a passivation layer which consists of the material of the insulating layer. It is disadvantageous that the aforementioned metallic alloys have a relatively high TCR value.
“Strain sensitive Pt—SiO2 nano-cermet thin films for high temperature pressure and force sensors”, H. Schmid-Engel, S, Uhlig, U. Werner, G. Schultes, Sensors and Actuators A. Physical, Volume 206, 1 Feb. 2014, Pages 17-21, discloses thin-film high-temperature strain gauges made of Pt—SiO2 for example which have a high K factor and low temperature coefficient of resistance (TCR) and can be used in a temperature range of up to 300° C.
It is the object of the invention to further develop a sensor element with a substrate which comprises a strain-sensitive element in such a way that high temperature stability, a TCR close to zero and the highest possible K factor can also be realised in the high-temperature range.